Segment liquid crystal displays (LCD) are displays typically used in a large variety of applications, e.g. in heating ventilation and/or air conditioning units in a car to show for example temperature, levels of ventilation and/or conditioning, etc. to show application specific symbols on the segments of the segment liquid crystal display. Segment liquid crystal displays may also be used for example in cluster instrument units or in many other electronic devices such as watches, displays in cameras, etc.
The most common segment LCD is typically the twisted nematic display. The twisted nematic display consists of a nematic liquid crystal sandwiched between two glass plates. Inner surfaces of the glass plates are coated with a transparent metal oxide film, which acts as an electrode, and are used to apply voltages to control visibility of a segment in the segment LCD. Polymer alignment layers are placed on top of the electrodes, and polarizers are applied to a top and bottom surface of the nematic liquid crystal display. In twisted nematic display the polarizers and the polymer alignment layers are oriented perpendicular to each other. One front plane electrode and one backplane electrode are typically associated with a segment of the segment LCD to control the visibility of the segment. Static driven segment LCDs have only one backplane electrode and one front plane electrode associated with each segment that the segment LCD can display. A front plane electrode or a backplane electrode may be associated to multiple segments of the segment LCD in which case the segment LCD is a dynamically driven segment LCD. For segment liquid crystal displays with a large number of segments the dynamically driven segment LCD allows to reduce a number of electrodes controlling the segment LCD compared to the static driven segment LCDs. Dynamically driven segment LCDs may have typically at least two backplane electrodes and a plurality of front plane electrodes organized in a matrix.
The voltage applied to the front plane and backplane electrodes of the segment LCD should have no DC components. If a DC component exists, any impurity ions present will migrate towards the front plane or back plane electrodes, causing an electric field to persist also in absence of an applied voltage so that the segment in the segment LCD may be not made visible or invisible anymore. Therefore the voltages applied to the front plane and backplane electrodes are typically AC voltages arranged such that at least over a predetermined number of cycles no DC components may be present.
While static driven segment LCDs may be driven with AC voltages with only two discrete voltage levels because only one backplane electrode and one of the front plane electrodes are associated with the same segment, dynamically driven segment LCDs must be controlled with AC voltages with more than two discrete voltage levels. In fact the visibility of a segment in dynamically driven segment LCDs depends not only on a first differential voltage between the AC voltages applied to the front plane and backplane electrodes associated with said segment but also on a second differential voltage between the voltage applied to the front plane electrode associated with said segment and the voltage applied to another backplane electrode not associated with said segment. In order to make said segment visible, the first differential voltage may be kept for example above a first predetermined threshold while the second differential voltage may be kept below a second predetermined threshold.
Furthermore, when segment LCDs are viewed at an angle away from their optical axis, a contrast of the visible segment versus an invisible segment decreases, especially in dynamically driven segment LCDs with a large number of backplane electrodes. The contrast of the visible segment versus the invisible segment in the segment LCDs may be improved by the use of more than two discrete voltage levels in the AC voltages controlling the front plane and backplane electrodes of the segment LCDs.
Segment LCDs are typically controlled by microcontrollers with an application specific interface to drive multiple backplane and front plane electrodes with AC voltages having multiple discrete voltage levels. For abating costs, segment LCDs may be typically controlled also by general purpose microcontrollers without the application specific interface. The application specific interface may in fact increase chip area if integrated on chip with the microcontroller or may increase component count if implemented off-chip. However one of the limitations of general purpose microcontrollers is that only two output discrete voltage levels are available to control the segment LCD: normally a supply voltage and a reference voltage (i.e. typically a ground voltage) of the general purpose microcontroller.
Application note AN3219 from Freescale “Xgate Library: TN/STN LCD driver” discloses a configuration of a general purpose microcontroller controlling a segment LCD with three discrete voltage levels which drive corresponding backplane electrodes. The third discrete voltage level is derived externally to the general purpose microcontroller by means of the supply voltage of the general purpose microcontroller connected to a resistor ladder. When the third discrete voltage level is needed, the two output discrete voltage levels of the general purpose microcontroller are disabled and corresponding outputs of the general purpose microcontroller at which the first two voltages are generated, are configured as inputs showing high impedance to the resistor ladder. When the third discrete voltage level is not needed, the corresponding outputs of the general purpose microcontroller override the resistor ladders and the general purpose microcontroller applies the two output discrete voltages levels (i.e. as said the supply voltage and the reference voltage of the general purpose microcontroller) to the corresponding backplane electrodes.
One problem associated with the solution proposed in the above mentioned Freescale application note is that said solution lacks of flexibility because no more than three discrete voltage levels can be produced. The contrast improvement between visible and invisible segments is thus limited. Furthermore use of the general purpose microcontroller is thus limited to applications with a limited number of backplane electrodes. It is in fact well known and also described in the above mentioned application note, that for applications with a high number of backplane electrodes, even an higher number of discrete voltage levels is needed in order to optimize the contrast between visible and invisible segments in segment LCDs.